Replace broken slowrx dependency with pure Python SSTV decoder

slowrx is a GTK GUI app that doesn't support CLI usage, so the SSTV
decoder was silently failing. This replaces it with a pure Python
implementation using numpy and Pillow that supports Robot36/72,
Martin1/2, Scottie1/2, and PD120/180 modes via VIS header auto-detection.

Key implementation details:
- Generalized Goertzel (DTFT) for exact-frequency tone detection
- Vectorized batch Goertzel for real-time pixel decoding performance
- Overlapping analysis windows for short-window frequency estimation
- VIS header detection state machine with parity validation
- Per-line sync re-synchronization for drift tolerance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/js/modes/sstv.js

@@ -183,11 +183,11 @@ const SSTV = (function() {
             Settings.registerMap(issMap);
         } else {
             // Fallback to dark theme tiles
-            L.tileLayer('https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png', {
-                maxZoom: 19,
-                className: 'tile-layer-cyan'
-            }).addTo(issMap);
-        }
+            L.tileLayer('https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png', {
+                maxZoom: 19,
+                className: 'tile-layer-cyan'
+            }).addTo(issMap);
+        }
 
         // Create ISS icon
         const issIcon = L.divIcon({
@@ -491,7 +491,7 @@ const SSTV = (function() {
 
             if (!data.available) {
                 updateStatusUI('unavailable', 'Decoder not installed');
-                showStatusMessage('SSTV decoder not available. Install slowrx: apt install slowrx', 'warning');
+                showStatusMessage('SSTV decoder not available. Install numpy and Pillow: pip install numpy Pillow', 'warning');
                 return;
             }